1. Field of the Invention
The present invention relates to a radar apparatus for detecting the direction of a target based upon reflected electromagnetic waves, and in particular to a radar apparatus having a plurality of receiving antennas and an array transmitting antenna, or a plurality of transmitting antennas and an array receiving antenna.
2. Background of the Invention
Types of radar apparatus are known having a transmitting antenna which transmits a beam of electromagnetic waves, with the direction of a target being detected based upon phase differences between resultant reflected electromagnetic waves received from the target. In the case of a phase monopulse radar apparatus, the reflected waves are received by two receiving antennas, with the direction of a target being detected based on the phase difference between reflected waves from the target that are respectively received by adjacent receiving antennas.
A phase monopulse radar apparatus has an advantage of high accuracy of direction detection, but has a disadvantage that the size of the range within which directions of targets can be detected is restricted. Considering a phase monopulse radar apparatus having a pair of receiving antennas (e.g., respective single antenna elements) spaced apart with a pitch L, designating the wavelength of the radar waves as λ, and the phase difference between respective received signals of the two receiving antennas (phase difference between reflected waves that are respectively received by the receiving antennas) as Δφ, the direction θ of a target which reflects the waves can be obtained from the following equation:θ=(180/π)sin−1{(Δφ/360)·(λ/L)}
Hence if Δφ is within the range −180° to +180° then the corresponding direction θ can be unambiguously obtained from Δφ. The range of directions for which unambiguous direction detection can be achieved is determined by the pitch L of the receiving antennas.
If values of Δφ exceed that unambiguous detection range, phase foldover (aliasing) occurs, i.e., it is impossible to distinguish between two targets whose respective directions correspond to phase differences of Δφ and (Δφ+360°).
For example, designating the respective directions of two targets as θ1 and θ2, and designating the phase difference (between the respective received signals of the two receiving antennas) corresponding to θ1 as Δφ1 and the phase difference corresponding to θ2 as Δφ2, and assuming that the phase difference Δφ2 is outside the aforementioned range of 180° to +180° while Δφ1 is within the range, then the direction that is calculated as θ1 will be correct while the direction that is calculated as θ2 may differ from the actual direction.
Prior art types of phase monopulse radar apparatus have therefore had the disadvantage that target directions can only be detected within a small range of directions, to avoid effects of phase foldover.
To overcome this problem, it has been proposed for example in International Patent publication No. WO99-34234 (referred to in the following as reference document 1) to use a radar apparatus having a plurality of transmitting antennas having respectively different directions of transmitted beam, with switching being applied for transmitting radar waves from these transmitting antennas in succession. Resultant reflected waves from a target are received by two receiving antennas, and the direction of the target is detected the phase difference between respective reflected waves reaching the receiving antennas. Since reflected waves from a target are received along respectively different directions from a plurality of transmitting antennas, a greater amount of information can be obtained than is possible by using only a single transmitting antenna. The information is used to authenticate each detected target, as described in the following, to enable the target detection range to be widened while maintaining high reliability of target direction detection.
Basically, if the detected direction of a target is sufficiently close to the orientation direction of the currently transmitted beam (e.g., is within the beamwidth of the transmitted beam that is currently being transmitted) then it is judged that an actual target is detected, since reflected waves generally return along a direction which does not differ greatly from the orientation direction of the transmitted beam. Hence, if the detected direction is not sufficiently close to the direction of the transmitted beam (i.e., is not within a predetermined range that is centered on the beam direction) the target is (provisionally) assumed to be false, i.e., caused by phase foldover.
Since detected targets can thereby be reliably authenticated, it becomes possible to use a phase monopulse type of radar apparatus having a wide target detection range within which phase foldover occurs.
The problem caused by phase foldover is thereby overcome by using a plurality of transmitting antennas.
However, such a type of radar apparatus is undesirable from a practical aspect. It is preferable to utilize an array antenna formed of an array of antenna elements as the transmitting antenna, and electronic scanning. Here, “electronic scanning” signifies that shifting of the beam to successive directions (i.e., successively shifting the beam to different orientation directions, to cover successively different direction ranges) is performed by phase control of the transmission signals supplied to the elements of the array. Specifically, the direction of the transmitted beam is varied by successively varying the phase difference between transmission signals applied to adjacent antenna elements.
Such an electronic scanning method is preferable in particular for a radar apparatus which is installed in a vehicle for example, since it enables the transmitting antenna (and hence the overall radar apparatus) to made compact.
If such electronic scanning of the transmitted beam were applied, then in principle it would be possible to eliminate the problem of ambiguity caused by phase foldover, since the orientation direction of the transmitted beam is known at each point in time, and that information can be used in authenticating each detected target as described above.
However if such an electronic scanning method were simply to be directly applied to a phase monopulse of radar apparatus for enabling target directions to be detected within a range in which phase foldover occurs as described above, then problems arise due to grating lobes in the beam pattern of the transmitting antenna. In particular, at least one grating lobe adjacent to the main lobe may be of similar magnitude to the main lobe. As the pitch of the elements of an array transmitting antenna is increased, the main lobe of the antenna beam is accordingly made narrower and the antenna gain is increased. However as the pitch is increased, the magnitudes of the grating lobes become increased and the grating lobes are brought closer to the main lobe, thereby, conversely, lowering the antenna gain.
Thus if the pitch is not appropriate, the direction of a grating lobe may come within the target direction detection range, as the direction of the transmitted beam is scanned through that range. Direction detection reliability (target authentication reliability) is thereby lowered, since (due to phase foldover, i.e., aliasing) a false target which results from reflected waves originating from a grating lobe may appear to have a direction that is close to the direction of the transmitted beam (i.e., direction of the main lobe).
For that reason, it has not hitherto been practicable to apply an electronic scanning array antenna to be used as a transmitting antenna in a radar apparatus, when targets are to be detected within a range of directions whereby phase foldover occurs.